An off-grid generator set is a power generating system whose connection to an electrical power grid is temporarily or permanently unavailable. Off-grid generator sets have been widely used. For example, a portable/standby generator set may be utilized to power electrical equipment connected via a circuit with the generator set when people do camping, tailgating and working outside. A similar power generating apparatus is also utilized to provide backup power in emergencies, for example during blackout.
The off-grid generator set usually utilizes a driving engine coupled to a generator (or alternator) through a common shaft. Upon actuation of the engine, the engine rotates the common shaft to drive the alternator that, in turn, generates electrical power. Most residential electric equipment is designed to be used in connection with electrical power having a fixed frequency, for example, 60 hertz (Hz) in North America and 50 Hz in China. The frequency of the output electrical power is primarily determined by the operating speed of the engine. As a result, the engine's operating speed of some generator sets is fixed to keep the frequency of the output electrical power fixed. However, when an electrical load applied to the generator set is less than the rated kilowatt load for which the generator set is designed, the engine's fuel-efficiency deteriorates.
Additionally, an off-grid generator set usually generates an output voltage at a certain level since most electrical loads are designed to work with a voltage at such level. For example, some household appliances uses a voltage of 120 volts and others uses a voltage of 240 volts.